Yellow and red phosphors can be used to create white light emitting GaN-based LEDs. These phosphors are typically encapsulated by a silicone. The latter material tends to yellow or darken over time. Moreover, due to its lower refractive index relative to that of the phosphor, the resultant backscatter degrades the LED efficiency. Previous work has demonstrated the efficacy of a glass encapsulant with low characteristic temperatures, particularly one that is index-matched to the phosphor as described in commonly-assigned U.S. patent application Ser. No. 13/281,671 filed on Oct. 26, 2011, the content of which is incorporated herein by reference in its entirety. However, the index-matched glasses developed to date for this application have typically contained Pb, which has since been deemed an inadmissible component.
Accordingly, there is a need for comparable Pb-free encapsulant glasses that can be index-matched to the LED phosphors and that can function as a sealing frit at temperatures below the upper use limit of the phosphors.